A particular problem exists in the production of polymer with removing residual unpolymerized monomer or solvent. Heating to the vaporization temperature of the monomer or solvent results in a gradual dissipation of the monomer over an extended period. An increase in temperature increases the monomer or solvent removal rate, unfortunately the rate of monomer generation also increases with temperature. However, as currently practiced, not only do long residence times increase costs of commercial products, but also result in physical property deterioration of the polymer and undesirably high amounts of residual monomer remain even after many hours. Residual monomers have many deleterious effects on a polymer. For example, they lower the strength characteristics, produce internal bubbles, may impart undesireable taste and odor to food materials in contact therewith, and reduce resistance to environmental conditions.
Thermoplastics are prepared by the polymerization of a suitable monomer or mixture of monomers, generally in the presence of a volatile liquid media. The initial product is produced in a solution, mass, emulsion, or suspension from which it is necessary to isolate the thermoplastic polymer from its unpolymerized monomer and solvent.
In particular, a wide variety of methods and apparatus have been employed to remove small amounts of volatile material from synthetic thermoplastic resins. However, many synthetic resinous thermoplastic materials are heat sensitive and tend to degrade on exposure to elevated temperatures. Thus in many instances, it is highly desirable to perform an operation upon the thermoplastic composition in such a manner that the composition is held at an elevated temperature for only a minimum length of time. Conventional devolatilization procedures are often undesirable.
Conventional devolatilizers are generally based upon a gravity feed system, i.e., typically the polymer will pass through a heat exchanger into a vacuum chamber and in so doing the volatile will essentially be removed from the polymer. The volatile-free polymer then falls under the force of gravity toward a pumping device which has the capability of removing the polymer from the vacuum chamber.
The gravity feed system, while providing satifactory results, has a limitation in that there is no flow control of the polymer. Consequently, the polymer may become hung up on the devolatilizer walls or may become trapped in stagnant or recirculating pools. In either instance, certain quantities of the polymer may be in the devolatilizer for a length of time sufficient to cause polymer degradation.
Attempts to correct this by varying the devolatilization temperature have not proven successful. If low temperatures are employed, the length of time the polymer spends in the chamber is excessive due to the relatively high viscosity of the polymer. The relatively high polymer viscosity causes the rate of polymer removal from the devolatilizer to be too slow for practical operation; further, due to reduced vapor pressure, the amount of volatile removal is substantially decreased. If high temperatures are employed, undesired degradation of the polymer results, which is generally in proportion to the temperature of processing and the length of time the polymer has been held at the elevated temperature. Substantial and significant difficulties arise with such heat sensitive polymers in that retention of small portions of a thermally degradable plastics composition often results in severe local decomposition and can result in black spots of charred resin which can render a mass of polymer totally unsuitable for the preparation of thin transparent film. Additionally at relatively high temperatures, although an increased volatility of the monomer causes an initial lowering of monomer content at the entrance of the devolatilizer, the monomer regeneration rate becomes so great that during the even shorter residence time the total amount of volatiles in the polymer is increased.
It would be desirable if there were available an improved method for the devolatilization of synthetic resinous materials which are readily thermally degraded.
It would also be beneficial if there were available an improved method for the devolatilization of thermally degradable synthetic resinous materials which material is produced into articles such as films, wherein the articles had been subjected to a minimal heat history.
A still further object of this invention is to provide a method which does not alter the chemical-physical properties of the resulting thermoplastic.
Another object of this invention is to provide an apparatus for the devolatilization of synthetic resinous materials which are readily thermally degraded.
Other and further objects of the invention will be apparent from the following description and claims and may be understood by reference to the accompanying drawings, which by way of illustration show preferred embodiments of the invention. Other embodiments of the invention may be used without departing from the scope of the present invention as set forth in the appended claims.